Implantable electronic medical devices serve many functions. A common function is to provide electrical stimulation to a particular muscle or organ of the body and/or to monitor the body's own natural electrical stimulation of that particular region. For example, an electronic pacemaker functions to provide electrical stimulation pulses, at a controlled rate, to selected chambers of the heart in order to ensure that the heart beats. Similarly, an implantable cardioverter-defibrillator (ICD) provides electrical stimulation pulses to selected chambers of the heart. The stimulation pulses from an ICD are much stronger and less frequent than those produced by pacemakers and are intended to correct rapid, uncoordinated heart beats (fibrillation).
In order to perform its intended functions, an implantable electronic medical device must have an energy source, e.g., a battery. Because the device is implanted in a patient and often performs life sustaining functions, the battery must be extremely reliable and should last as long as possible in order to delay the surgery required to explant or replace the device. Additionally, to prevent harm to the patient from failure of the medical device due to failure of the battery, most such electronic medical devices are configured to provide a signal to the patient or physician indicating that the end of life (EOL) of the battery is approaching. This signal is also referred to as the recommended replacement time (RRT) notification (or signal) for the device.
A well-known characteristic of implantable electronic medical devices, in particular implantable cardioverter-defibrillators (ICDs) and pacemakers, is that the mix of chemicals of the battery used within the device changes as the battery gets older and more depleted, thereby causing the output voltage of the battery to decrease. This characteristic is in fact used to help predict when it is time to replace the device with one having a new battery. Thus, it is common to have the ICD or pacemaker regularly monitor the terminal battery voltage, or output voltage. If the output voltage of the battery drops below a preset threshold, the recommended replacement time (RRT) signal, or equivalent flag or trigger, is initiated.
A number of different types of batteries are known and used in implantable electronic devices, for example lithium iodine batteries, lithium silver chromate and silver vanadium oxide (SVO) batteries. These different battery types have different characteristics that may be useful for different purposes. Increasingly, SVO batteries are being chosen by manufacturers for use in certain implantable electronic devices, particularly implantable cardioverter-defibrillators.
Most ICDS, pacemakers and other implantable electronic medical devices manufactured today include one or more microprocessors. These microprocessors operate most efficiently and reliably under conditions of steady, predictable voltage from the energy source. Because silver vanadium oxide (SVO) batteries maintain a relatively flat voltage during most of their later life, they are particularly well suited to use in these devices. Unfortunately, this advantageous feature of SVO batteries is also disadvantageous. The disadvantage is that it is difficult to predict, using voltage alone, when the battery will fail and thus at what voltage threshold the recommended replacement time (RRT) signal should be activated.
SVO batteries, as well as other batteries, have an additional disadvantage, particularly when used in ICDs. The primary function of an implantable cardioverter-defibrillator (ICD) is to sense the occurrence of an arrhythmia, and to automatically apply an appropriate shock therapy to the heart aimed at terminating the arrhythmia. For example, if the ICD senses that the patient's heart is fibrillating, i.e., beating in a rapid, uncoordinated manner, then the ICD automatically delivers a high current shock to the patient's heart to defibrillate the organ. ICDs typically operate by first detecting the arrhythmia, then rapidly charging one or more storage capacitors contained within the device, and then quickly discharging the capacitor(s) to deliver the life saving shock therapy. Rapidly charging a capacitor, however, creates a severe load on the battery. If the battery's internal impedance is high, the battery is unable to provide adequate voltage to quickly, fully charge the capacitor. This can result in a delay in the delivery of the defibrillating shock or even in complete failure of the ICD.
The internal impedance of a battery is dependent not only upon the age of the battery, but also upon its usage history. For example, if the battery is discharged rapidly and/or frequently, its internal impedance rises very little. If, on the other hand, the battery is discharged very slowly and is not used to deliver a shock for a number of months, the internal impedance may increase very significantly. Thus, relying on the age of the implanted device or measuring only its terminal battery voltage in order to determine the recommended replacement time for the device provides an inaccurate indication of the remaining longevity of the battery. This is particularly true where the device is an ICD which may be called upon to perform its intended function(s) only infrequently.
Measuring terminal battery voltage not only inadequately accounts for the battery's internal impedance, it also can result in a significant waste of battery life where the battery has a flat voltage during the latter stages of its life, such as do SVO batteries, and where a battery voltage replacement threshold is selected that is near or the same as such flat voltage. In order to avoid the risk of failure of the SVO battery, ICD manufacturers carefully and conservatively select the RRT signaling voltage threshold. Generally, manufacturers set this voltage threshold between about 2.485 and 2.55 volts per cell. While these thresholds help ensure that the device will be replaced prior to battery failure, they can also result in a significant loss of valuable ICD usage, because a substantial amount of energy may yet remain in the battery when the output voltage is at these conservative thresholds.
What is needed is a simple, efficient, safe means of determining the recommended replacement time for an implantable electronic medical device, particularly an ICD, that is protective of patient safety yet maximizes battery energy usage.